In conventional TV transmitting and receiving systems there are many cases where image signals are transmitted in a restricted band. For example, in the NTSC system used for the broadcast, the bands are 4.2 MHz for the brightness signal E.sub.Y, 1.5 MHz for the color difference signal E.sub.I, and 0.5 MHz for E.sub.Q.
Taking the brightness signal as an example, rectangular wave signals in the neighborhood of f=1 MHz and 2 MHz in the transmission band of 0.about.4.2 MHz indicated by (a) in FIG. 4 (corresponding to clear and dark stripes in an original image) are displayed as follows. Rectangular wave signals indicated by (b) are transformed into those indicated by (c) in the same figure by the band restriction. In particular, for f=2 MHz, only the sinusoidal wave, which is the fundamental wave, is displayed as the brightness signal in the reproduced pattern.
As described above, for the rectangular wave (stripe pattern), which is the fundamental wave of the brightness signal at f=2 MHz, heretofore only the sinusoidal wave, which is the fundamental wave, is displayed, as indicated by (c) in FIG. 4. Due to this fact, sharpness of the pattern is lost. Further the band for E.sub.Q is 0.5 MHz and since for the rectangular wave at 0.2 MHz only the fundamental wave is displayed, image quality is worsened.
On the other hand, for transmission recording of audio signals there are bands suitable therefor. For example, suitable transmission bands are 0 to 15 kHz for the FM broadcast and 0 to 20 kHz for CD. Designating the upper limit frequency of these transmission band by f.sub.C, this can be considered as a kind of low pass filters (LPF), whose cutoff frequency is f.sub.C. In the case where the frequency of a sound source is below f.sub.C as indicated by (a) in FIG. 11, there is no problem, even if the transmission is effected through the low pass filter (LPF) described above, because it is recorded or transmitted as it is.
However, in the case where the sound source has components over f.sub.C, as indicated by (b) in FIG. 11, it is transmitted or recorded without the components over f.sub.C, which are removed by the low pass filter (LPF) described above, as indicated by (c) in FIG. 11. Therefore, of course, the components over f.sub.C are not reproduced and thus sound different from the original is reproduced.
When sound sensing techniques become excellent and performance of reproducing apparatuses is improved, as they are recently, if components over 15 kHz for FM and over 20 kHz for CD are removed, worsening in sound quality may be felt. In general, it is said that the audible sound region is 20 Hz to 20 kHz, but sensitivity is not zero at 20 kHz.
Heretofore the reproduction of audio signals was aimed under the restriction in the band by the transmission system described above. However, when the original sound source has a frequency distribution f.sub.A as indicated by (b) in FIG. 11, since the transmitting system or the recording system is restricted in the transmission band as described above, audio signals pass through a low pass filter, whose cutoff frequency is f.sub.C, and the signals of f.sub.A &gt;f.sub.C are removed. As the result, even if they are reproduced by a reproducing system with a high fidelity, the components of f.sub.A &gt;f.sub.C cannot be reproduced. This gives rise to a problem from the point of view of a high fidelity reproduction.
That is, since the band of f.sub.A &gt;f.sub.C thus removed includes frequency components giving rise to rich high frequency sounds, if these components are cut by a transmitting system equivalent to an LPF, as they were heretofore, there is a problem that high fidelity reproduction of the original sound is impossible and sound quality is worsened.